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NO2 Emission Increases Associated with the Use of Certain Diesel Particulate Filters in Underground Mines

In response to new exposure standards to lower miners’ exposure to diesel particulate matter (DPM), the National Institute for Occupational Safety and Health (NIOSH) and others have conducted research into control technologies to reduce DPM emissions. The mining industry—and specifically dieselized mines—also continue to work toward finding feasible controls to implement in their mines. Although emissions of and exposure to DPM can sometimes be controlled through the use of newer diesel engines, better engine maintenance, use of alternative fuels, or ventilation upgrades, some mines may need to use diesel particulate filters. However, this has created concern about potential exposure to nitrogen dioxide (NO2) resulting from their use.

NO2 is a deep lung irritant and the Mine Safety and Health Administration (MSHA) has set a ceiling value (a concentration that shall not be exceeded even instantaneously during a shift) of 5 parts per million (ppm). NO2 is emitted from a naturally aspirated diesel engine at a concentration of between 50 and 100 ppm. These concentrations are subsequently diluted by mine air resulting in lower concentrations in the workplace air. However, the use of diesel particulate filters can cause up to a three-fold increase in the concentrations of NO2 emitted from the tailpipe, with a corresponding increase in the concentration of NO2 in the work area.

Figure 2: Inlet and outlet of a diesel particulate filter showing the accumulation of diesel particulate matter on the inlet side.

Diesel particulate filters have been shown to be highly efficient in filtering DPM, removing up to 99% of DPM from diesel engine emissions. Figure 1 shows a schematic of the basic concept of filtering where engine emissions (blue arrows) enter at one end of the filter and then must physically flow through the filter wall, which removes the DPM, before exiting the filter. Figure 2 presents a picture of both the inlet and outlet of a filter showing that the black diesel soot is only seen on the inlet side of the filter. Disposable and nondisposable filters have been successful in efficiently removing DPM on mining equipment. After use, the disposable filters are simply removed and thrown away. Disposable filters have not been shown to cause an increase in NO2 tailpipe emissions.

However, nondisposable filters must have a way of removing the DPM that has accumulated on the filter, a process called regeneration. Active regeneration uses an external heat source to burn off accumulated DPM. For example, the filter can be physically removed from the equipment and the DPM burned off in an oven. Once this process is complete the filter can be reinstalled on the equipment. Diesel particulate filters relying on this off-line regeneration have not been shown to cause an increase in NO2 tailpipe emissions.

A second regeneration process is termed passive regeneration. This type of regeneration occurs as the engine is operating and typically uses the heat generated by the engine to complete the regeneration. Often, this type of regeneration uses an oxidation catalyst, such as platinum, vanadium or similar compound, to reduce the temperature necessary to begin regeneration. These catalysts can be used as either a fuel additive or as a washcoating. The catalyst in the form of a fuel additive is added to the diesel fuel prior to combustion, whereas, a catalyst in the form of a washcoating is used to coat the walls of a filter during the manufacturing process, creating a highly catalyzed filter system. Although the presence of a catalyst, in either form, will increase the feasibility of the filter by allowing regeneration at lower exhaust temperatures, it may also result in an increase in the concentration of NO2 in the exhaust. NIOSH research has not found an NO2 emissions increase associated with the use of catalysts as fuel additives. However, recent NIOSH studies have shown a two- to three-fold increase in NO2 concentrations in mine air, resulting from the use of highly catalyzed filter systems. Therefore, the use of highly catalyzed filters on underground equipment must be closely monitored. Of course, NIOSH has not tested all catalysts used with filters. Consequently, mines should be aware of the chemical composition of any catalysts they choose to employ and the resulting impact on NO2 emissions.

The increase in NO2 emissions resulting from the use of highly catalyzed filters on engines of model year 2007 or newer on-highway pick-up trucks is also a concern in the mining industry. Western coal mines use these trucks as transportation and utility vehicles inside the mines. NIOSH recommends the use of newer modern engines as part of the solution to controlling DPM in underground mines. Environmental Protection Agency (EPA) data for these trucks show a significant reduction in tailpipe emissions of DPM and nitrogen oxide (NOx) when compared to previous year models. However, NIOSH recently cooperated in a study that showed if these pick-up trucks are tested at several engine modes from the MSHA 8-mode test, the NOx emissions actually increase over previous model year levels. An increase in NOx emissions along with the use of a highly catalyzed filter will result in a correspondingly greater increase in NO2 emissions. These data suggest that using pick-up trucks with a model year later than 2007 in underground mines may create elevated NO2 concentrations in the workplace. NO2 concentrations should be continuously monitored when these trucks are used to ensure a safe workplace.

As the mining industry continues to look for ways to reduce the concentration of diesel particulate matter in underground mines, they also need to consider the effects that these controls may have on NO2 concentrations. Although NIOSH is aware of many DPM control strategies, the discussion presented here focuses only on NO2 increases associated with the use of highly catalyzed filters in underground mines. Mines implementing any control strategies must be mindful of their potential to increase NO2 emissions.

NIOSH will continue its research on control technologies that both reduce DPM and NO2 concentrations and will present these results to the industry. As a means of improving this research and technology transfer, NIOSH requests the assistance of the mining community by informing us of their successes and failures related to DPM control strategies.

More information on NIOSH diesel research can be found at the NIOSH mining webpage.

Steven Mischler, PhD, Emanuele Cauda, PhD

Mr. Mischler is the Acting Manager of Diesel and Dust Monitoring in the NIOSH Respiratory Hazards Control Branch in the Pittsburgh Research Laboratory.
Dr. Cauda is a Research Fellow in the NIOSH Respiratory Hazards Control Branch in the Pittsburgh Research Laboratory.

96 comments on “NO2 Emission Increases Associated with the Use of Certain Diesel Particulate Filters in Underground Mines”

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The research is excellent and the paper describes it well. But the authors should have chosen a more accurate title. NO2 increases were seen with one type of diesel particulate filter—highly catalyzed filters with on-board regeneration. No problems were found with other filters. Yet the title would lead one to believe that NO2 increases were found with diesel particulate filters in general. This is not just a copy-editing issue.

Small operators may read the title and conclude that they cannot use filters at all. And sadly, some in the mining industry continue to resist DPM controls, and will cite the title in their public relations.

You raise a good point. We have changed the title of the blog to better reflect our research findings that indicate a two- to three-fold increase in NO2 concentrations in mine air resulting from the use of highly catalyzed filter systems. The new title reads “NO2 Emission Increases Associated with the Use of Certain Diesel Particulate Filters in Underground Mines.”

I am a graduate student at the University of Miami in a Public Health course. I read with interest the passage on NO2 emissions with diesel filters in mines. As with any intervention, unexpected side effects may be encountered that must be addressed. There exists evidence in the literature of success with tocopherols in detoxifying NO2 within ex vivo cells. Has there been any thought to use of tocopherols or other detoxifying agents to convert the nitrogen dioxide to a less toxic form?

In other words, would a chemical reaction within the exhaust system distal to the filters be helpful in removing the harmful NO2 from the air?

We thank you for your comment. To my knowledge no research has evaluated the use of tocopherols, or other biological chemicals, for converting Nitrogen dioxide (NO2) to a less toxic form. However, chemical reactions are extensively used in aftertreatment technologies to control NO2 and NOx emissions from diesel-powered applications. These technologies convert NO2 to N2 via a reduction reaction over a catalytic surface. The reduction is performed by a reductant species injected into the exhaust gases. The Selective Catalytic Reduction (SCR) strategy employs ammonia or urea as the reductant species while the Lean NOx Catalyst (LNC) uses the injection of hydrocarbons to perform this conversion. Another possible control approach is to absorb NO2 onto a catalysts’ active sites and then convert it to N2 during fuel-rich engine operation conditions (NOx adsorbers). All of these technologies have been studied in the past and developed for on-highway and off-highway surface applications, however the implementation of these emission control technologies into an underground mine presents unique challenges including a demanding work environment, limited space on the equipment and equipment accessibility.

Small operators may read the title and conclude that they cannot use filters at all. And sadly, some in the mining industry continue to resist DPM controls, and will cite the title in their public relations.

Thank you for the comment. We agree that the introduction and adoption of advanced DPM aftertreatment strategies by the mining industry is still fragmentary. NIOSH’s role is to publish correct and fair information in order to explain the benefit and the threats connected with the use of a technology. The title of this blog well describes the possible increase of NO2 connected with the use of only CERTAIN and not ALL Diesel Particulate Filters.

As the mining industry continues to look for ways to reduce the concentration of diesel particulate matter in underground mines, they also need to consider the effects that these controls may have on NO2 concentrations. I think you’ve made some great points here and I look forward to hearing more.

I am hoping I can get some experienced feedback from the professionals who have been there and done it and are currently doing it in the workforce as of today. Or any great advice will be greatly appreciated!

As the mining industry continues to look for ways to reduce the concentration of diesel particulate matter in underground mines, they also need to consider the effects that these controls may have on NO2 concentrations. it is very problemative issue and unfortunatly we are not able to deal with it .

Since DPM is classified as carcinogenic to humans, controlling DPM is important also in the construction industry. Diesel Particulate Filters have high efficiency in reducing DPM at the tailpipe of diesel engines. The blog describes hazards from exposure to NO2 from underground mining use of DPM controls. The confined space operation and ventilation configuration (i.e., workers in tunnels downwind from machinery) for operation of diesel equipment in an underground mine is, with limited exceptions, different from those typically encountered in the construction sector. On the surface, exhaust is usually directed away from nearby workers. This combined with the higher circulation and dilution intrinsically present in any surface operation would reduce potential exposure to NO2. For this reason, the higher emission of NO2 connected with the use of highly catalyzed filters may be less of a concern. Nevertheless specific studies could be done by the Construction Sector.

Thanks for posting this article. It is really helpful for research students.
TiO2 nano Particles is helpful to control the NO2 emission. Ti nano particles in TiO2 stoichiometry help to retart NO2 emission.

With increase in the pollution, we must search for the sustainable and pollution less source of energy.
NO2 emission should be controlled with latest technology.
Thanks for the articel.
Hopefully NIOSH will continue its research on control technologies that both reduce DPM and NO2 concentrations.

In my opinion one of the ways of addressing this issue is to ensure that there’s enough awareness of the importance of treated fuel or diesel. Untreated diesel results in incomplete combustion which leaves behind soot, NOT, CO, and THCs (total hydrocarbons). These can pose a lot of health and environmental challenges.

There is a body of evidence that would suggest that any increase in NO2 can be reduced/managed if the temperature across catalyzed surfaces is managed and consistent? That is, the deliberate creation of non-laminar (i.e. very turbulent) exhaust flows reduces any increase in NO2. Having a DOC in front of the PM filter and reducing the availability of HO also reduces the post combustion formation of NO2 from NO.

In my opinion one of the ways of addressing this issue is to ensure that there’s enough awareness of the importance of treated fuel or diesel. Untreated diesel results in incomplete combustion which leaves behind soot, NOT, CO, and THCs (total hydrocarbons). These can pose a lot of health and environmental challenges.

This is an interesting, in-depth article about the NO2 emissions. Thank you for bringing this concern to the forefront. The health and safety of everyone exposed to these emissions must be prioritized.

If somebody reads only the title may conclude that they cannot use filters at all. And sadly, some in the mining industry continue to resist DPM controls, and I’m afraid they will cite the title in their public relations.

This is an interesting, in-depth article about the NO2 emissions. Thank you for bringing this concern to the forefront. The health and safety of everyone exposed to these emissions must be prioritized.

Thank you for your comment. The National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Mining Research Division (PMRD) currently has a team of several researchers working to reduce exposures of miners to aerosols and gases emitted by diesel powered vehicles and equipment in underground mines. The primary focus of these efforts includes:
• Development and evaluation of technologies and strategies to prevent overexposures to DPM of critically affected occupations such as blasters, load-haul-dump (LHD) operators, drillers, truck drivers, and scallers in underground metal and nonmetal mining operations.
• Evaluation and implementation of novel and emerging advanced engine technologies for heavy- and light-duty underground mining applications.
• Development and evaluation of canopy air curtains for mobile underground mining equipment as a control strategy for preventing exposure to diesel aerosols.
• Development and evaluation of filtration and pressurization systems for environmental enclosures for mobile underground mining equipment as a control strategy for preventing exposure to diesel aerosols.
• Development and evaluation of advanced disposable filter elements for use in filtration systems for permissible diesel-powered equipment.
• Development and refinement of methodologies for off- and on-line monitoring of exposure of underground miners to diesel aerosols.

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